The present invention is to a process for producing ferric-aminopolycarboxylic acid chelates with improved long term stability.
In the photographic industry, the oxidation of metallic silver in photographic images or negatives to silver ion is known as bleaching. A desirable bleaching agent will react rapidly with silver and then react rapidly with air to regenerate the bleaching agent. Ferric compounds have been used for this purpose for decades. The ferric compound in the most widespread use today is ferric ammonium ethylenediaminetetraacetate because of its desirable redox properties and ease of preparation from inexpensive commercial chemicals such as, ferrosoferric oxide, ethylenediaminetetraacetic acid (EDTA) and ammonia.
Donovan and Surash, U.S. Pat. No. 3,767,689, and Svatek, et al. U.S. Pat. Nos. 4,364,871, and 4,438,040, describe the formation of ferric-aminopolycarboxylic acid chelates by the reaction of iron oxide with ammoniated EDTA in an aqueous mixture at temperatures up to 105.degree. C. for less than three hours, followed by pH adjustment, aeration, and filtration to give a ferric ammonium EDTA solution suitable for bleaching. A more recent variation of the method, described by Thunberg, et al. (U.S. Pat. No. 5,110,965), involves the use of ferrous salts to catalyze the reaction between the iron oxide and ammoniated EDTA.
Whereas chelate solutions made by the aforementioned processes are useful for photographic bleaching, over time they deposit fine, black, particulate matter which would be detrimental to the quality of photographs prepared using said solutions. It would be desirable, therefore, to produce ferric ammonium EDTA solutions which are more stable against the formation of the dark particulates.